The present invention relates to a photothermographic material, in particular, a photothermographic material for scanners and image setters, which is suitable for photomechanical processes. More precisely, the present invention relates to a photothermographic material for photomechanical processes that can provide images showing low fog, high Dmax (maximum density) and little increase of fog during storage.
A large number of photosensitive materials are known which have a photosensitive image-forming layer on a support and form images by exposing imagewise. Among such materials, as an example of a system that contributes to environmental protection or enables simplification of image formation means, there is a technique of forming an image by heat development.
In recent years, reduction of amount of waste processing solutions is strongly desired in the field of photomechanical processes from the standpoints of environmental protection and space savings. Therefore, techniques relating to photothermographic materials for use in photomechanical processes are required to be developed, which enables efficient exposure by a laser scanner or a laser image setter and formation of a clear black image having high resolution and sharpness. Such photothermographic materials can provide users with a simpler and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
Methods for forming images by heat development are described in, for example, U.S. Pat. Nos. 3,152,904, 3,457,075 and D. Klosterboer, Imaging Processes and Materials, xe2x80x9cThermally Processed Silver Systems Axe2x80x9d, 8th ed., Chapter 9, page 279, compiled by J. Sturge, V. Walworth and A. Shepp, Neblette (1989). Such a photothermographic material contains a reducible non-photosensitive silver source (e.g., silver salt of an organic acid), a photocatalyst (e.g., silver halide) in a catalytically active amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. The photosensitive material is stable at an ambient temperature, but when the material is heated at a high temperature (e.g., 80xc2x0 C. or higher) after light exposure, silver is produced through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the reducible silver salt in the exposed region shows black color and this presents a contrast to the non-exposed region to form an image.
In many of conventionally known photothermographic materials, an image-forming layer is formed by coating a coating solution using an organic solvent such as toluene, methyl ethyl ketone (MEK) and methanol as a solvent. However, not only use of an organic solvent as a solvent adversely affect human bodies during the production process, but also it is disadvantageous in view of cost because it requires process steps for recovery of the solvent and so forth. Accordingly, methods of forming an image-forming layer by coating a coating solution using water as a solvent have been proposed. For example, Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) 49-52626, JP-A-53-116144 and so forth disclose image-forming layers utilizing gelatin as a binder, and JP-A-50-151138 discloses an image-forming layer utilizing polyvinyl alcohol as a binder. Furthermore, JP-A-60-61747 discloses an image-forming layer utilizing gelatin and polyvinyl alcohol in combination. As another example, JP-A-58-28737 discloses an image-forming layer utilizing a water-soluble polyvinyl acetal as a binder. If these binders are used, image-forming layers can be formed by using a coating solution comprising an aqueous solvent, and therefore considerable merits can be obtained with respect to environment and cost.
However, when a polymer such as polyvinyl alcohol or water-soluble polyacetal is used as a binder, silver tone of developed areas becomes brown or yellow, which quite differs from black color regarded as a preferred proper color, and in addition, there arises, for example, a problem that the blacking density in exposed areas is low and the density in unexposed areas is high. Thus, there can be obtained only those of which commercial value is seriously impaired. Further, there is a problem that fog is more likely to be caused and in particular, increase of fog is more significant during storage in a photosensitive material based on the aforementioned heat development system compared with the conventional chemical treatment type photosensitive materials. Furthermore, chemical sensitization with a gold sensitizer suffers from a drawback that it is extremely likely to cause fog, while it can provide high sensitivity. Therefore, there has been desired a technique for providing a photothermographic material that shows high sensitivity, can provide images showing low fog, high Dmax (maximum density) and little increase of fog during storage, and is advantageous for environment and cost by utilizing a binder usable in the aforementioned aqueous solvent type coating solution.
For use of photographic art films in the fields of newspaper printing, commercial printing and so forth, there have generally been desired systems that can provide stable images at any time. However, photothermographic materials showing such high-contrast photographic property as mentioned above, which is required for films for photomechanical processes, suffer from a problem that they show higher temperature and humidity dependency during development compared with conventional films to be treated with chemicals, and thus white line width or dots are likely to come to narrow or small at a high temperature or under high humidity, or density may decrease or white line width may become large at a low temperature or under low humidity. Therefore, as for photothermographic materials, it has been desired to provide a photothermographic material that shows low temperature and humidity dependency during development and thus is suitable for use in photomechanical processes.
The present invention was accomplished in view of the aforementioned various problems, and its first object is to provide a photothermographic material that shows high sensitivity, low fog, high Dmax (maximum density), little increase of fog during storage and low temperature and humidity dependency during development, as a photothermographic material for photomechanical processes, in particular, for scanners and image setters. The second object of the present invention is to provide a photothermographic material that can be produced by coating of aqueous system, which is advantageous for environment and cost.
The present invention provides a photothermographic material containing at least a photosensitive silver halide, a non-photosensitive silver salt of an organic acid, a reducing agent for silver ions and a binder on one surface of a support, wherein the material contains at least one compound satisfying at least one of (i) to (iv) and an organic gold compound and the photosensitive silver halide has a mean grain size of 0.12 xcexcm or less:
(i) a compound producing imagewise a chemical species that can form development initiation points on and in the vicinity of the non-photosensitive silver salt of an organic acid (except for hydrazine derivatives);
(ii) a compound that provides increase of developed silver grain density to a level of 200-5000% when it is added in an amount of 0.01 mol/mol of silver (except for hydrazine derivatives);
(iii) a compound that provides increase of covering power to a level of 120-1000% when it is added in an amount of 0.01 mol/mol of silver (except for hydrazine derivatives);
(iv) a compound represented by any one of the following formulas (1) to (3): 
In the formula, R1, R2 and R3 each independently represents a hydrogen atom or a substituent, Z represents an electron-withdrawing group, and R1 and Z, R2 and R3, R1 and R2, or R3 and Z may be combined with each other to form a ring structure. 
In the formula, R4 represents a substituent. 
In the formula, X and Y each independently represent a hydrogen atom or a substituent, A and B each independently represents an alkoxy group, an alkylthio group, an alkylamino group, an aryloxy group, an arylthio group, an aniline group, a heterocyclyloxy group, a heterocyclylthio group or a heterocyclylamino group, and X and Y or A and B may be combined with each other to form a ring structure.
The organic gold compound used for the photothermographic material of the present invention preferably consists of at least one compound represented by any one of the following formulas (4) to (6).
xe2x80x83[Au(L)2]+Xxe2x88x92xe2x80x83xe2x80x83Formula (4)
In the formula, L represents a ligand, two of L may be identical to or different from each other, and at least one of L represents a mesolon ligand. Xxe2x88x92 represents an anion.
[L-Au-L]Mxe2x80x83xe2x80x83Formula (5)
In the formula, L represents an organic mercapto ligand and M represents a cationic counter ion, provided that this complex has a symmetrical form.
[(M-Rsol)n-A-Sxe2x80x94Auxe2x80x94S-A-(Rsol-M)n]Mxe2x80x83xe2x80x83Formula (6)
In the formula, M represents a cationic counter ion, Rsol represents a hydrophilic group, A represents a substituted or unsubstituted divalent organic bridging group, n represents any of 1-4, and when n is 2 or larger, n of (Rsol-M) may be identical to or different from each other or one another, provided that the compound has a symmetrical form.
The photosensitive silver halide used for the photothermographic material of the present invention preferably consists of at least one compound represented by the following formula (7). 
In the formula, Xxe2x80x2 each independently represents xe2x80x94Oxe2x80x94, xe2x80x94NHxe2x80x94 or xe2x80x94NRxe2x80x94, R represents an alkyl group, a fluoroalkyl group, an aryl group or a sulfonyl group, and m and r each represent 0, 1 or 2, provided that m and r do not simultaneously represent 0. M represents hydrogen or a cationic species, Ar represents an aromatic group, L2 represents a bridging group, and p represents 0 or 1. In the formula, (m+r) of Xxe2x80x2, M, L2 or p as well as two of Ar may be identical to or different from each other or one another.
The present invention also provide an image formation method comprising subjecting the aforementioned photothermographic material to light exposure for 10xe2x88x926 second or less and heat development to form an image, an image formation method comprising subjecting the aforementioned photothermographic material to light exposure utilizing a multi-beam heat development apparatus provided with two or more laser heads and heat development to form an image, and an image formation method comprising subjecting the aforementioned photothermographic material to light exposure and heat development at a line speed of 140 cm/minute or more to form an image.